This invention relates to desorption and ionization methods and apparatuses to produce gas phase ions for subsequent analysis. More particularly, it relates to ionization of gaseous analytes subsequent to adsorption of the gaseous analyte to an ionization surface.
This invention generally relates to methods and apparatuses for the adsorption, desorption, and ionization of an analyte for analysis of the ionized analyte by such analytical methods as, for example, mass spectrometry.
How analytes are ionized depends on the volatility of the analyte. That is, volatile analytes are typically ionized into the gas phase, by methods such as electron ionization (EI), chemical ionization (CI), or photoionization by lasers. Involatile analytes are either desorbed from surfaces by energy input or desorbed in liquid sprays and detected as ions. Desorption from surfaces occurs in methods such as laser desorption (LD), secondary ion mass spectrometry (SIMS), and matrix-assisted laser desorption (MALDI). Desorption from liquid sprays occur in methods, such as electrospray (ES) and thermospray (TSP).
These methods of analyte ionization produce a variety of both positive and negative analyte ions. Positive ions include molecular ions (M+), protonated molecules (MH+), cationized molecules (A+(M)), and various fragment ions (F1+). Negative ions include molecular ions (Mxe2x88x92), deprotonated molecules ((Mxe2x88x92H)xe2x88x92), anionized molecules (Xxe2x88x92(M)), and fragment ions (Fxe2x88x92).
The positively and negatively charged analyte ions can then be extracted from the ion source by an electric field, and separated according to their m/z ratios, using magnetic sector, quadrupole, time-of-flight, Fourier-transform, ion trap, or other types of mass spectrometers. The molecular identity of the analyte can usually be determined from the measured mass-to-charge ratios of the structurally significant ions.
Some ionization methods involve deposition of the analyte on a surface. In LD, analytes are deposited on a surface, usually metal, which is then irradiated with a laser to produce structurally significant ions for lower molecular weight materials (generally less than about 1,000 Da). In MALDI, which is primarily used for larger analytes, such as proteins, analytes are deposited on a surface together with a large excess of matrix. Isolating the analyte in the matrix is considered necessary in order to observe analyte ions. In SIMS, analytes are deposited on a surface, which is then bombarded with keV-kinetic energy primary ions, which cause secondary ions to be emitted from the surface. Another ionization method that involves surfaces is surface-assisted laser desorption ionization (SALDI). In SALDI laser irradiation is used to desorb ions from suspensions of solid carbon powders in a liquid matrix and from beds of carbon powders immobilized on a substrate. Here it is believed that the individual particles protrude above the surface of the liquid matrix.
Yet another surface ionization method is xe2x80x9chot wirexe2x80x9d surface ionization. In this method, gaseous molecules are ionized in the vicinity of a hot (ca. 600xc2x0 C.) filament. This method gives primarily fragment ions and the surface topography is not a factor.
Another surface ionization method set forth in U.S. Pat. No. 6,288,390 is desorption ionization from porous silicon (DIOS) wherein analyte ions are obtained by irradiating a porous silicon substrate loaded with analyte. The silicon substrate is described as having a porosity, measured gravimetrically, from approximately 4% to substantially 100%, with 60-70% porosity most preferred. The porous silicon substrate is formed by chemical etching with an ethanol/hydrofluoric acid mixture. Analyte is adsorbed or loaded onto the porous substrate prior to inserting the loaded substrate into the analyzing instrument where the substrate is irradiated, which then causes desorption of ionized analyte. The porosity of the substrate surface is critical to the ability to load the substrate with analyte molecules.
When gas phase analytes are analyzed, the analyte typically is maintained in the gas phase for both the ionization process and subsequent analysis. However, where the gaseous analyte has associated with a substrate surface prior to ionization such as in, for example, field ionization (FI) and field desorption (FD) methods, the ionization of the interacted gaseous analyte is accomplished by application of electric voltage to the substrate itself. Critical to this method is the presence of very sharp edges and tips on the substrate. The electric potential applied to the substrate creates extremely high electric fields at such tips and edges. The ionization of the analyte is a direct result of these high electric fields. Application of external sources of energy, such as laser light, onto the surface has limited or no effect on the mass spectra obtained.
Thus, except for FI, FD, and surface ionization, where ionization occurs by application of either an electric field or temperature in the immediate vicinity of the surface, mass spectrometry procedures used to analyze gases and gaseous mixtures rely on the ionization in the gas phase of the analyte.
All of the above described procedures for producing ionized particles are limited by low ionization efficiency for structurally significant ions. In the gas phase, ionization efficiency is determined by the cross section for the elementary ionization process, the flux of ionizing particles or photons, and the time that the gaseous analyte molecules spend in the ionization region. For the most commonly used ionization methods (electron ionization and chemical ionization) the fraction of analyte molecules ionized is approximately 10xe2x88x924. In the electron capture method, typically, used for high electron-affinity compounds, the ionization efficiency is higher, about 10xe2x88x922. However, few if any fragment ions are usually formed by this method. While an ionization efficiency close to 1 can be achieved with photoionization, it requires very high power lasers and mainly forms atomic ions. Those ions contain very little structural information, which makes identifying more complex molecules virtually impossible.
Mass spectrometers, such as magnetic sector or quadrupole instruments, are almost always used with continuous ionization where analyte ions are continually being formed and analyzed by the appropriate mass spectrometers. Commonly these instruments have a disadvantageous low ion detection efficiency particularly when analyzing ions with a wide range of m/z values. The low ion detection efficiency results because only ions within a limited range of m/z values can be detected at any one time.
In contrast, other types of mass spectrometers, such as time-of-flight instruments, have high ion detection efficiencies, that is, detection of essentially all ions formed. Efforts have been made to use such high ion detection efficient mass spectrometers for the analysis of gaseous compounds by using either a pulsed ionization scheme, or ion storage devices, or a combination thereof. Presently, among the approaches that are not limited to selected compounds, the highest sensitivity method is probably gas phase ionization with subsequent analysis in an ion trap.
Despite the very high sensitivity of current mass spectrometric methods for gas phase analysis, there remains a need to detect even smaller amounts of analytes. Such detection would provide for improved monitoring of gas purity, detection and quantification of trace compounds in the atmosphere, and ultra-sensitive gas chromatographic detection.
There further remains a need for an ionization method that utilizes a microscopically rough surface of a solid substrate to promote in situ adsorption of analyte, ionization, and desorption of the ionized analyte to achieve both increased ionization efficiency of gaseous analytes and increased detection efficiency of the ions formed. Such a method would be an advancement over the known methods. More particularly there remains a need for a method that utilizes in situ adsorption of gaseous analyte to a surface, followed by ionization, and desorption of an ionized gas phase analyte for analysis.
The present invention is directed to a method and a device for producing an analyte ion, comprising providing a substrate having a non-porous rough surface; contacting an analyte with the non-porous rough surface whereby the analyte interacts with the non-porous rough surface; and upon exposure of the non-porous rough surface to an energy source producing an ionized gas phase analyte.
The inventive said non-porous rough surface is structured to interact with the analyte. More specifically, the non-porous rough surface is structured to promote one or all of the following: adsorption of the analyte onto the surface; formation of ionized analyte on the surface; and desorption of ionized analyte from the surface.
The inventive method could also have the analyte in the gas phase prior to contacting with the non-porous rough surface. The analyte could be gasified prior to contacting with the non-porous rough surface. Thus, the analyte could be originally in the liquid or solid phase prior to a gasification process to produce the desired gaseous analyte. The analyte could also be a gaseous eluate from a gas chromatograph. The analyte could also be present in ambient air, sampled to contact the non-porous rough surface. The gaseous analyte can be added, for example, by either a gas injector or as a gas stream directed towards the non-porous rough surface.
Another embodiment of the present inventive method could also have the analyte in the gas phase prior to contacting with a substrate. The analyte could be gasified prior to contacting with the substrate. Thus, the analyte could be originally in the liquid or solid phase prior to a gasification process to produce the desired gaseous analyte. The analyte could also be a gaseous eluate from a gas chromatograph. The analyte could also be present in ambient air, sampled to contact the substrate. The gaseous analyte can be added, for example, by either a gas injector or as a gas stream directed towards the substrate.
The present invention is ideally suited for use as a highly efficient ion detection device where ions are produced by exposure to a pulsed energy source, such as a laser. The present invention can accumulate analyte then, upon exposure to a laser pulse, desorb gas phase ions. Following the desorption of gas phase ions the surface continues the process of accumulating analyte again. Such a pulsed method can be advantageously used with various existing analytical techniques, including, without limitation, gas chromatography and TOF mass spectrometry.
The surface features of the non-porous rough surface are critical to the invention and are sub-micrometer surface features, generally smaller than about 0.1 xcexcm. Overall the non-porous rough surface has a surface roughness of between about 10 nm and about 100 nm.
The present inventive substrate comprises at least one member of the group consisting of silicon, carbon, and polymers. Preferably, the substrate is single crystal silicon, or highly oriented pyrolytic graphite. The substrate could also be made from UV or IR light-adsorbing polymers, such as, for example, polystyrene. The non-porous rough surface could also be supported on low heat conductivity material.
There are various methods of roughening the surface of the substrate using a surface roughening treatment. Examples of possible surface roughening treatments are etching with reactive chemicals, bombardment with hyperthermal reactive atoms, bombardment with high-energy particles, irradiation with lasers, exposure to a plasma, vapor deposition, and roughening with mechanical action. The roughening treatment should produce a surface with the desired roughness features on a non-porous surface. The roughening treatment must not render porous the substrate or surface on which the rough features are formed.
The present method can also include a method of analyzing a physical property of the ionized gas phase analyte. Such analysis could be performed by any of the following: mass spectrometry, ion mobility spectrometry, and a current measurement device. Any number of analytical methodologies that measure physical properties of ions could be utilized in the present method.
A matrix could also be added to the non-porous rough surface to further interact with the analyte. Possible matrix materials include, for instance, water, glycerol, and acetic acid. The matrix could be added to the non-porous rough surface by adsorption of gas phase matrix material. The addition of the matrix could occur both before and after exposing the non-porous rough surface to an energy source. The addition of the matrix to the non-porous rough surface could occur in situ or in a different place than the place where exposure of the substrate to an energy source occurs. Additionally, the addition of the matrix to the non-porous rough surface could occur simultaneous with contacting the gaseous analyte to the non-porous rough surface.
The exposure of the non-porous rough surface to an energy source may be accomplished by irradiating the surface with laser light. Preferably, the surface is exposed to an energy source in a pulsed manner.
The substrate could also be cooled prior to contacting the analyte with the non-porous rough surface. Preferable, the substrate is cooled to a range of about xe2x88x92140xc2x0 C. to about xe2x88x9280xc2x0 C. Most preferably, the substrate is cooled enough to cause formation and reformation of condensed matrix vapor, for example, water vapor, on the substrate between laser pulses.
The irradiation of the non-porous rough surface can occur with light of a wavelength absorbed by either of the non-porous rough surface or a matrix added to the non-porous rough surface. Preferably, UV or IR light produced by a laser is utilized. Particularly, 337 nm UV laser and/or 3.28 xcexcm IR laser light is used for desorption of the absorbed analyte from the non-porous rough substrate surface.
The inventive method can be performed under ambient pressure or low pressure. Low pressure means from about 10xe2x88x924 to about 10xe2x88x926 torr, or typical pressures seen in a TOF mass spectrometer, for example.
The contacting of an analyte to the non-porous rough surface can occur in situ, that is, at the same location, where the non-porous rough surface is exposed to an energy source. Thus, where the exposure to an energy source is accomplished by means of, for instance, laser pulses, and the gaseous analyte is present in situ, then the gaseous analyte contacts the substrate surface both before and after exposure of the non-porous rough surface to a pulsed energy source. Hence, the analyte may be contacting the substrate surface to effectively regenerate coverage on the surface between laser pulses.
The present invention is also directed to an apparatus to perform the above described process, or more specifically, a device for generating analyte ions, which features a substrate having a non-porous rough surface; means for exposing an analyte to the non-porous rough surface whereby the analyte interacts with the non-porous rough surface; and an energy source to supply energy at the non-porous rough surface to generate ionized gas phase analyte.
Additional features, advantages, and embodiments of the invention may be set forth or apparent from consideration of the following detailed description, drawings, and claims. Moreover, it is to be understood that both the foregoing summary of the invention and the following detailed description are exemplary and intended to provide further explanation without limiting the scope of the invention as claimed.